The key thing to understand is that Marvel is CHEAP thrifty and they would rather give opportunities to obviously talented but underused directors and actors than spend a fortune on someone that everyone knows is good.

Plus they have a pre-existing relationship with Joss on Runaways and Astonishing X-Men.

A lot of what Marvel is doing is convincing people to do their films for less (and signing them to multiple film deals) because being in a "cool" comic-book movie that makes a ton of dough will help their careers.